·Ovarian cyst- This is the most frequent cause of a cystic lower abdominal mass in

the female fetus. The ovarian cysts may be ‘simple’ or theca lutein.

The cysts may be unilocular or multiseptate. Large cysts up to

10 cm in diameter are occasionally seen.

·Meconium psudocyst- These develop in response to the

presence of meconium secondary to bowel perforation in utero. Perforation of the fetal bowel may bethe result of bowel atresias/stenosis, volvulus, intussuception

cystic fibrosis, often resulting inperitoneal calcification.

·Mesenteric cyst- Mesenteric cysts are thought to be lymphatic in origin and are reported to occur most commonly in the mesentery of the small bowel. Sonographically they usually have the appearances of a unilocular cystic mass but rarely a multiseptate appearance is seen.

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·Choledochal cyst- Usually seen in the subhepatic region, usually unilocular but may rarely have a multiseptate appearance. If the gallbladder is visible the cyst usually appears separate from it. Contiguous tubular cystic structures may suggest the diagnosis.

·Retroperitoneal lymphangioma- Abdominal lymphangiomas most commonly involve the mesentery but are occasionally seen in the omentum or retroperitoneum. The majority of lymphangiomas are left sided. Lymphangiomas are usually multilocate, although a unilocular cystic appearance has been described.

·Hepatic mesenchymal hamartoma- Mesenchymal hamartomas of the liver usually present as a solitary round lesion with well defined margins consisting of multiple cysts of variable sizes separated by thick or thin septae.

·Urachal cyst- Persistence of the urachal lumen without communication with the bladder or umbilicus may result in urachal cyst formation. These are seen as cystic structures adjacent to the dome of bladder behind the anterior abdominal wall.

·Cystic teratoma- Teratomas may be ovarian or retroperitoneal and present a whole spectrum of US appearances. They may be solid, complex or cystic. If cystic they may be unilocular or multiseptate.

·Enteric duplication cyst- Enteric duplication cysts may occur at any point in the GIT. The stomach is less frequently affected than other regions. Duplication cysts are normally seen as thick walled fluid collections, particularly if they do not communicate with the lumen of the bowel. They may occasionally be multiseptate.